(1) Field of the Invention
The present invention relates to an array antenna calibration apparatus and an array antenna calibration method. The invention relates particularly to a technique for calibrating phase differences at array antenna ends.
(2) Description of the Related Art
Digital cellular radio communication systems employing the DS-CDMA (Direct Spread Code Division Multiple Access) technology have been developed as next-generation mobile communication systems. The CDMA scheme is an access scheme in which channels are assigned according to codes to make simultaneous communication available. In CDMA, signal interference of other channels used in simultaneous communication causes a problem of a limited number of channels available in simultaneous communication, thereby causing a limited channel capacity. To increase the channel capacity, techniques for restraining interference are effective.
An adaptive array antenna, which forms a beam for a desired user while it forms a null point for another user who becomes a significant source of interference, is an art for increasing the channel capacity. That is, the adaptive array antenna forms a beam in the direction of the desired user, and it directs a null point in the direction of the user who becomes a significant source of interference. This makes it possible to receive a radio wave from the desired user with high sensitivity, and not to receive a radio wave from the significant interference source, so that the amount of interference is reduced, thereby increasing the channel capacity.
Adaptive array antennas generate beams utilizing phase differences at antenna ends. Thus, phase variation in each radio unit will make it impossible to correctly control beam patterns.
Accordingly, correct control of beam patterns will necessitate correction of the phase difference at each antenna end. As a phase difference correction method, for example, calibration signals are multiplexed, and the phase difference of the multiplexed signals is detected and corrected.
For example, FIG. 9 is a block diagram showing an example of an array antenna calibration apparatus, and it is equivalent to FIG. 1 of the following patent document 1. The conventional apparatus of FIG. 9 includes: antenna elements 100-1 through 100-8 constituting a linear antenna; transmitters 103; a calibration signal generator 104; adders 105; circulators 106; a receiver 107; an RF switch 108; a calibration factor calculating unit 109; multipliers 110; a power combiner 111; a user signal multiplexing unit 112; beam formers 113 one for each user “1” through “n”. User signals sent from the beam formers 113 are multiplexed by the user signal multiplexing unit 112. After that, each multiplier 110 multiplies the multiplexed signals by a calibration factor obtained by the calibration factor calculating unit 109, and then each adder 105 adds a calibration signal generated by the calibration signal generator 104. The resultant signals are input to the transmitters 103 and sent out from the corresponding antenna elements 100-1 through 100-6. The antenna elements 100-7 and 100-8, one on each side of the array antenna, are dummy antennas to each of which a non-reflection resistor 102 is coupled.
Here, the signals sent from the antenna elements 100-1 through 100-6 are electromagnetically coupled to the adjacent antenna elements and transmitted. These coupled components are taken out by the circulators 106 and are then received by the receiver 107 via the RF switch 108.
For example, calibration signals C1 and C3 sent from the antenna elements 100-1 and 100-3, respectively, are received by the antenna element 100-2 due to electromagnetic coupling between the antenna elements, and signals C1+C3 are taken out by the corresponding circulator 106 and are then input to one of the ports of the RF switch 108. In the similar manner, signals C2+C4, signals C3+C5, and signals C4+C6 are input, one to each of the other ports of the RF switch 108. Here, signals C3 and C5, electromagnetically coupled to the antenna elements 100-1 and 100-6, are power-synthesized by the power combiner 111 and are then received by the receiver 107 via the RF switch 108.
After that, the ports of the RF switch 108 are sequentially changed over, and the signal input to each port is demodulated and converted into a baseband signal by the receiver 107. The calibration factor calculating unit 109 measures the phase and the amplitude of each calibration signal to calculate a calibration factor. For example, signal patterns orthogonal to one another with no correlation therebetween are used as calibration signals C1 through C6, and signals C1 and C3 are subjected to correlation processing by the corresponding signal patterns of the signals C1 and C3, to obtain the phases and the amplitudes of the signals C1 and C3, and a factor for making uniform the amplitudes and the phases of the signals C1 and C3 is obtained. Likewise, the ports of the RF switch 108 are sequentially changed over, and factors for making uniform the amplitudes and the phases of signals C2 and C4, signals C3 and C5, signals C4 and C6, and signals C2 and C5 are individually obtained.
Next, from the thus obtained factors, calibration factors for making uniform the phases and the amplitudes of all the signals C1 through C6 are obtained, and the multipliers 110 multiply transmission signals by these calibration factors, thereby making it possible to make uniform the amplitudes and the phases of the signals sent from the antenna elements 100-1 through 100-6.
In addition, another conventional technique is disclosed in the following patent document 2. This conventional technique calibrates the phases and the amplitudes of antenna elements based on a component, coupled to each antenna element, of calibration signals sent from additive antennas disposed, one on each side of an array antenna and on a user signal received by each antenna element. This makes it possible to allow for the characteristics of a transmission path from the antenna elements to the receiver, and an array antenna calibration apparatus in which a positional relationship between a base station and a signal generator need not be acknowledged is realized.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-218621
[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-92508
However, in both of the above conventional arts, the phase differences among calibration signals are detected on the assumption that intervals between antenna elements are already known. Hence, a problem is that antenna element interval deviation will cause calibration-error.